plot "data.txt" w l title "standard function",\
     "data_5.txt" w l title "interpolate with 5 points",\
     "data_10.txt" w l title "interpolate with 10 points",\
     "data_15.txt" w l title "interpolate with 15 points",\
     "data_20.txt" w l title "interpolate with 20 points"
set title "plot of C"
set grid xtics ytics ls 100
set grid linecolor rgb "#808080"

set term png
set output "./image/C.png"

replot

set output